feat(gpu): afegir suport SPIRV (Vulkan) per Linux/Windows

- Nou: shaders/sprite.vert|frag, postfx.vert|frag, ball.vert (GLSL)
- Nou: cmake/spv_to_header.cmake — converteix .spv → uint8_t C header
- CMakeLists.txt: bloc non-Apple troba glslc, compila GLSL → SPIRV en
  build-time i genera headers embeguts a build/generated_shaders/
- gpu_context.cpp: MSL|METALLIB en Apple, SPIRV en la resta
- gpu_pipeline.cpp: createShaderSPIRV() + branques #ifdef __APPLE__
  per sprite/ball/postfx pipelines
- Corregeix crash a engine.cpp:821 (Windows/Linux) causat per pipelines
  null quan init() fallava en no trobar suport MSL

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shaders/ball.vert

@@ -0,0 +1,23 @@
+#version 450
+// Per-instance data (input_rate = INSTANCE in the pipeline)
+layout(location=0) in vec2 center;
+layout(location=1) in vec2 halfsize;
+layout(location=2) in vec4 col;
+layout(location=0) out vec2 v_uv;
+layout(location=1) out vec4 v_col;
+void main() {
+    // gl_VertexIndex cycles 0..5 per instance (6 vertices = 2 triangles)
+    // Vertex order: TL TR BL | TR BR BL  (CCW winding)
+    const vec2 offsets[6] = vec2[6](
+        vec2(-1.0, 1.0), vec2(1.0, 1.0), vec2(-1.0,-1.0),
+        vec2( 1.0, 1.0), vec2(1.0,-1.0), vec2(-1.0,-1.0)
+    );
+    const vec2 uvs[6] = vec2[6](
+        vec2(0.0,0.0), vec2(1.0,0.0), vec2(0.0,1.0),
+        vec2(1.0,0.0), vec2(1.0,1.0), vec2(0.0,1.0)
+    );
+    int vid = gl_VertexIndex;
+    gl_Position = vec4(center + offsets[vid] * halfsize, 0.0, 1.0);
+    v_uv  = uvs[vid];
+    v_col = col;
+}